Steam stimulation of petroleum-bearing formations, or reservoirs, has become one of the preferred methods of enhanced oil recovery. This is because steam is cost-effective to supply heat to low-gravity, high viscosity oils. Heat reduces resistance of oil flow from a reservoir to a producing well over a wide range of formation permeabilities. Further, such steam injection enhances the natural reservoir pressure, above that due to the hydrostatic head, or depth-pressure gradient, to increase the differential pressure between oil in the reservoir and the producing well bore.
The producing well may be the same well through which steam is periodically injected to stimulate petroleum flow from the reservoir (popularly called "huff and puff"). Alternatively, one or more producing wells may be spaced from the injection well so that the injected steam drives petroleum through the reservoir to at least one such producing well.
Almost all earth formations forming petroleum reservoirs are created by sedimentary deposition, with subsequent compaction or crystallization of the rock matrix. Such deposition of detrital materials, with varying composition and over extensive geological times, occurs at varying rates. The resulting compacted rocks in which petroleum accumulates are permeable, but in general the flow paths are quite heterogeneous. Accordingly, a petroleum reservoir formed by such rock formations are inherently inhomogeneous as to both porosity and permeability for fluid flow of either native (connate) or injected fluids. Furthermore, flow permeability for connate gas, oil and water is substantially different for each liquid or mixture. Because of these differences in permeability, it is now common practice to inject foam forming surfactants with the injected steam to block the more permeable gas passages that may develop in the formation. The desired result is to divert steam from the more permeable gas passageway to less permeable oil-rich zones of the reservoir. The foaming component is usually an organic surfactant material
Another particular feature of such inhomogeneity of sedimentary rock formations appears to be their shale or clay content It is known that such clay material is susceptible to alteration when contacted by water and particularly when the injected water is in the form of steam having little salt content. In general, clays have large surface area to volume ratios and when altered by water or steam tend to affect adversely contact between connate oil and reservoir rock. Most specifically, reservoirs containing viscous oils having organo-metallic and acidic components are particularly susceptible to both steam and surfactant materials used in enhanced oil recovery. Further, the solubility of surfactant in the connate water of the formation and the reservoir oil may have a deleterious effect on the foaming ability or stability of the foam where gas permeability and steam loss has increased by fingering or gas override. Thus, the foam may not be active where residual oil in the highly permeable channels exceeds more than a few percentage of the pore volume. Hence, there is a need for foam forming surfactant compositions which preferentially form foam in contact with residual oil within or around the steam- or water-permeable passageways of the formation, but without significant foam formation where they contact oil in oil-rich flow channels through the reservoir.
This invention is an improvement over prior methods of using foam-forming compositions to enhance petroleum production from oil-bearing formations. Many of these are mentioned and discussed in U.S. Pat. No. 4,086,964. Others include U.S. Pat. Nos. 4,393,937, 4,532,993 and 4,161,217. It is also an improvement over previously known methods of foam formation to control "fingering" or "over-ride" by injecting a foam- forming surfactant with gas or steam which primarily foams upon contact with residual oil portions of the reservoir created by gas or steam flow paths therethrough.
The need for surfactants which foam in the presence of both oil and water has been known for some time. Bernard ("Effect of Foam on Recovery of Oil by Gas Drive" Prod. Monthly 27. No. 1, 18-21, 1963) noted that the best foaming surfactants for immiscible displacements such as steam floods are those which foam when both oil and water are present. Dilgren et al. (U.S. Pat. No. 4,086,964) recognized the importance of non-condensable gas and added electrolyte such as sodium chloride for steam foams and discloses the use of an alkyl aromatic sulfonate, dodecylbenzene sulfonate, for this use. Other patents teach the use of alkyl aromatic sulfonates for this use without recognizing a difference in performance for the branched and linear structures (U.S. Pat. No. 4,532,993; 4,161,217 and 3,348,611). U.S. Pat. No. 4,161,217 teaches that mixtures of low molecular weight (300-400) and high molecular weight (400-600) alkyl aromatic sulfonates are useful foaming agents for hot water non-condensable gas foam drives. A still more recent patent (U.S. Pat. No. 4,562,727) teaches that olefin sulfonates such as alpha olefin sulfonates are substantial improvements over alkyl aromatic sulfonates. The present invention identifies a class of branched alkyl aromatic sulfonates which offer substantial advantages over the surfactants of the prior art to produce better foaming behavior in the presence of varying amounts of residual oil. They are especially useful for improving oil recovery from reservoirs with high permeability zones containing varying amounts of residual oil having pore volume saturations of from a few percent to 30% or higher.